The advantageous use of silane polymer materials, preferably electron-beam grafted to polyolefin and other polymer films, as for such purposes as bonding with barrier layers, has been described in earlier U.S. Pat. No. 4,803,126, of common assignees with the present invention. In copending patent applications Ser. Nos. 336,848 now U.S. Pat. No. 5,077,135, filed Apr. 12, 1989, and 557,521 filed Jul. 24, 1990, of said common assignees, furthermore, there was reported the discovery that appropriate polysiloxane coatings applied upon such polymer film surfaces as polyethylene, polypropylene and the like, can provide for greatly decreased permeability in the films to such gases and oils and greases, and with preferred grafting of the coatings to the film surface with the aid of electron beam radiation techniques. Such techniques provide food packaging materials, for example, of polysiloxane polymer-coated films having improved gas impermeability, reduced aroma and flavor transmission properties and improved surface appearance and clarity, as described in said copending applications.
It is well known to those skilled in this art that to achieve strong adhesive bonds to the surface of polymers, polyolefins, such as polyethylene, polypropylene and the like, the film surface may be corona treated. It is believed that such corona treatment not only removes oils, greases and impurities from the surface of the film but, in addition, often causes oxidation of the surface, introducing carboxyl and carboxylic acid groups. In the previously mentioned patent and patent applications, it was shown that the coating of the polyolefin films with certain silanes, such as a vinyl benzylamine siloxane (Dow Corning Z6032), produces a coated surface that, following treatment of the coating with the electron beam, allows for greatly improved adhesion both to the surface of the polyolefin film and to adhesives. In the above-mentioned patent and patent applications, a process is described wherein, for example, Dow Corning Z6032 siloxane is dissolved in alcohol, hydrolyzed by the addition of water, and allowed to equilibrate into stable structures over a 12-24 hour period. The corona-treated polymer film (corona-treated primarily to remove grease, oil and dirt) is then treated with this alcohol solution and the alcohol solution is dried. It is believed that the resulting coating polymerizes to a somewhat cross-linked structure wherein the vinyl benzyl groups are at the interface between the polymer surface and the siloxane coating and additionally at the interface between the siloxane coating and air.
In the above-mentioned patent, we showed that prior to treating with electron beam irradiation, the siloxane coating can be treated with other unsaturated materials such as acrylated urethane adhesives and the like, and that the electron beam treatment then grafts the siloxane coating to the polyolefin film and additionally polymerizes and grafts the acrylated urethane coating to the siloxane. This process produces adhesive bonds with much greater strength than could be achieved be merely treating the corona-treated polyolefin film with conventional adhesives.
It has also been demonstrated that some of these unsaturated materials may be included with the hydrolyzed siloxane solution. For example, we have shown that the addition of monomeric acrylamide to the siloxane solution produces, after coating and drying and subjecting to electron beam treatment, a polyolefin article which has polyacrylamide grafted to it. This polyacrylamide coating is extremely hydrophilic and produces a polyolefin article which is very easily lubricated by water.
Similarly, and perhaps most importantly, we have now found that the addition of appropriate organic acids, and in particular unsaturated carboxylic acids, to the siloxane solution prior to coating of the polyolefin or other polymer film results in a coating in which we have deliberately introduced additional carboxylic acid groups at the air-coating interface. The advantage of this process is that many more carboxyl groups are introduced on the surface or at the air interface than can be achieved by simple corona treatment of the polyolefin or other polymer film itself. This is indicated by the fact that while the critical surface tension of a corona-treated polyolefin film is in the range of 35-40 dynes per centimeter, the coatings of the present invention in which we include such carboxylic acids have critical surface tensions well in excess of 60 dynes per centimeter.
It is known that the adhesion of evaporated metal layers such as aluminum to polyolefin film surfaces is greatly improved when the surface of the film is corona treated. Similarly, barrier polymer materials such as, for example, polyvinylidine dichloride applied in the form of a latex show improved wettability and adhesion on corona-treated films, compared with the non-corona-treated films, as described, for example, in the above-referenced patent. It is the purpose of our invention, therefore, to deliberately supply to the surface of the coated film many more carboxylic acid groups than can be achieved by simple corona treatment of a polyolefin or other polymer film, greatly improving the wetting and adhesion of all coatings, either organic or inorganic, applied to the surface of these films.
Altogether apart from such improved wettability and adhesion properties, however, it has also been discovered that the addition of unsaturated carboxylic acid to the siloxane monomer prior to coating on the polymer film has resulted, after polymerization and grafting to the polymer film, particularly by electron beam radiation, in a startlingly improved oxygen, gas, aroma, flavor and fragrance impermeability of the coated film.
Although both saturated and unsaturated carboxylic acids may be employed in the practice of this invention, unsaturated acids such as maleic acid, fumaric acid, citraconic acid and itaconic acid, among the carboxylic acids, are preferred because, under the influence of the electron beam radiation, they polymerize and graft to the siloxane coating, rendering these acids non-migrating in the presence of moisture or heat, and producing, surprisingly and unexpectedly, a very substantial improvement in the oxygen impermeability for the oxygen barrier properties of the siloxane film.